Mechanisms and Kinetics of Reaction‐Bonded Aluminum Oxide Ceramics

Abstract

Reaction-bonded A1,0, (RBAO) ceramics were fabricated starting from mechanically alloyed AI,O,/AI, AI,O,/ AI/ZrO,, and Al2O,/AI/ZrO,/Zr mixtures. Isopressed compacts were heat-treated in air up to 1550°C. Reactionbonding mechanisms, kinetics, and the influence of ZrO, and Zr additions are investigated. Independent of additive, oxidation of Al proceeds both as solid/gas and liquid/gas reaction, and the reaction kinetics follow a parabolic rate law. The reaction rate depends strongly on the particle size of AI. The activation energy of the reaction depends essentially on green density. Below the melting temperature of Al, in samples containing 45 vol% A1 and 55 vol% AI,O,, it is 112 and 152 kJ/mol at -64% and -74% TD, respectively, while above the melting temperature, it lies in the range -26-33 M/mol. Zr additions reduce the activation energy to some extent. Samples with only ZrO, additions exhibit nearly the same activation energies as Zr0,-free samples, though ZrO, has a very positive effect on the microstructural development in RBAO ceramics. Microstructure evolution and some strength data of RBAO bodies are also reported.

The dependence of the elastic moduli of reaction bonded alumina on porosity

“…4a. Though it is generally known that it is difficult to detect ␥-alumina due to the low intensity of the peaks, 27 the more probable interpretation is that the limited precision of the porosity measurements accounts for the relatively low values for the Young's modulus of the pure alumina phase.…”

Attrition milling of metallic-ceramic particles in acetyl-acetone

“…The aluminium oxidation examined here is similar to that studied previously on the RBAO, in which a similar oxidation temperature and DSC phenomena were reported. 19 As shown in Fig. 9, both samples showed a shrinkage from 200 to 400 C, which might be due to evaporation of Zr(acac) 4 and Al(acac) 3 adsorbed at the surface of the particles.…”

Tailoring protective coatings for all‐oxide ceramic matrix composites in high temperature‐/high heat flux environments and corrosive media

“…The RBAO layer is applied through a simple painting technique of a Al/Al 2 O 3 powder mixture dispersed in isopropanol followed by sintering at 1350 C in air. Reaction-bonding of alumina powders and Al metal is a wellexplored processing routine for alumina-based monolithic ceramics and composites [6,7] which has several advantages over conventional a-alumina powder metallurgy: (i) in-situ oxidation of Al metal accounts for lower processing temperatures while (ii) the sintering-induced shrinkage is partly or even fully compensated by the oxidation controlled volume expansion of the Al starting powder. Adherence of the green coatings on the substrate is excellent with no cracks or debonding of the RBAO/CMC interface occurring upon firing.…”